1. Field of the Invention
The present invention relates to well logging. In particular, the present invention is an apparatus and method for determining the resistivity of subsurface formations using electrical methods.
2. Background of the Art
In conventional galvanic resistivity measurement tools using a focusing technique, a guard electrode emits current in order to lead the current beam of a measurement electrode deeper into a conductive material. The resistivity of the material is determined by means of measurement electrode's voltage and current registration. The driving potential on guard and measurement electrode must be exactly the same to avoid disturbances of the ideal electrical field, which makes sure that the focusing effect takes place. Higher driving potential differences may lead to currents from guard to measurement electrode or vice versa passing the borehole fluid around the tool, which would completely destroy the focusing effect and lead to high measurement errors if not considered. In general, the focusing effect will lead to an electrical current with a higher penetration depth compared to that without focusing.
Birdwell (U.S. Pat. No. 3,365,658) teaches the use of a focused electrode for determination of the resistivity of subsurface formations. A survey current is emitted from a central survey electrode into adjacent earth formations. This survey current is focused into a relatively narrow beam of current outwardly from the borehole by use of a focusing current emitted from nearby focusing electrodes located adjacent the survey electrode and on either side thereof. Ajam et al (U.S. Pat. No. 4,122,387) discloses an apparatus wherein simultaneous logs may be made at different lateral distances through a formation from a borehole by guard electrode systems located on a sonde which is lowered into the borehole by a logging cable. A single oscillator controls the frequency of two formation currents flowing through the formation at the desired different lateral depths from the borehole. The armor of the logging cable acts as the current return for one of the guard electrode systems, and a cable electrode in a cable electrode assembly immediately above the logging sonde acts as the current return for the second guard electrode system. Two embodiments are also disclosed for measuring reference voltages between electrodes in the cable electrode assembly and the guard electrode systems
Techniques for investigating the earth formation with arrays of measuring electrodes have been proposed. See, for example, the U.S. Pat. No. 2,930,969 to Baker, Canadian Pat. No. 685,727 to Mann et al. U.S. Pat. No. 4,468,623 to Gianzero, and U.S. Pat. No. 5,502,686 to Dory et al. The Baker patent proposed a plurality of electrodes, each of which was formed of buttons which are electrically joined by flexible wires with buttons and wires embedded in the surface of a collapsible tube. The Mann patent proposes an array of small electrode buttons either mounted on a tool or a pad and each of which introduces in sequence a separately measurable survey current for an electrical investigation of the earth formation. The electrode buttons are placed in a horizontal plane with circumferential spacings between electrodes and a device for sequentially exciting and measuring a survey current from the electrodes is described.
The Gianzero patent discloses tool mounted pads, each with a plurality of small measurement electrodes from which individually measurable survey currents are injected toward the wall of the borehole. The measurement electrodes are arranged in an array in which the measurement electrodes are so placed at intervals along at least a circumferential direction (about the borehole axis) as to inject survey currents into the borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole. The measurement electrodes are made small to enable a detailed electrical investigation over a circumferentially contiguous segment of the borehole so as to obtain indications of the stratigraphy of the formation near the borehole wall as well as fractures and their orientations. In one technique, a spatially closed loop array of measurement electrodes is provided around a central electrode with the array used to detect the spatial pattern of electrical energy injected by the central electrode. In another embodiment, a linear array of measurement electrodes is provided to inject a flow of current into the formation over a circumferentially effectively contiguous segment of the borehole. Discrete portions of the flow of current are separately measurable so as to obtain a plurality of survey signals representative of the current density from the array and from which a detailed electrical picture of a circumferentially continuous segment of the borehole wall can be derived as the tool is moved along the borehole. In another form of an array of measurement electrodes, they are arranged in a closed loop, such as a circle, to enable direct measurements of orientations of resistivity of anomalies
The Dory patent discloses the use of an acoustic sensor in combination with pad mounted electrodes, the use of the acoustic sensors making it possible to fill in the gaps in the image obtained by using pad mounted electrodes due to the fact that in large diameter boreholes, the pads will necessarily not provide a complete coverage of the borehole.
The electrochemical equilibration process between a metal (e.g. electrode) and an electrolytic fluid leads to layers of complex resistive behavior. The impedance of these layers is called contact impedance. When current flows into or from an electrode, a difference between the potential immediately outside and inside the electrode will be created by the impedance layer. Contact impedances are highly variable and nonlinear. They depend mainly on electrode material, electrochemical properties of the fluid, current density and frequency of the applied voltages. In natural environments, the formation is filled with fluid whose chemical composition is not completely controllable, it is almost impossible to predict exactly contact impedances for measurement and guard electrodes. The focusing effect will be weakened or damaged whenever the potentials of guard and measurement electrodes beyond the impedance layer become different. This difference could cause a current to flow through the mud between the electrodes. The contact impedance thus has at least significant impact on the resolution of the measurement.
U.S. Pat. No. 6,373,254 to Dion et al. describes a method and apparatus to control the effect of contact impedance on a formation resistivity measurement during a logging-while-drilling operation. The control of contact impedance is accomplished by maintaining a substantially zero difference in potential between two monitor electrodes positioned on the resistivity logging tool near a current electrode.
Others have discussed the use of monitor electrodes for wireline applications. See, for example, Davies et al. (SPE24676), Evans et al. (U.S. Pat. No. 6,025,722), Seeman (U.S. Pat. No. 5,396,175), Smits et al. (SPE 30584), and Scholberg (U.S. Pat. No. 3,772,589). The monitor electrode technique uses two additional electrodes (called monitor electrodes) located between measurement and guard electrode to observe a possible potential difference. The potential on measurement or guard electrode is adjusted by means of a control circuit in order to keep the voltage between the monitor electrodes ideally at zero. Monitor electrodes emit no current and are therefore assumed to be unaffected by contact impedances. From the minimum voltage drop between the monitor electrodes it is concluded that the potential difference between guard and measurement electrodes each beyond the impedance layer immediately outside the electrode is zero.
FIG. 3 (prior art) is a schematic diagram of a typical resistivity imaging tool suitable for use in water-based mud. The formation is depicted by 151. The imaging tool includes one or more center electrodes (measurement electrodes) 157, one or more guard electrodes 155 which focus the measure current, and a return electrode 153, (usually the tool body). An AC voltage source is connected between the guard and return electrodes, resulting in a current flow from guard to return through the drilling mud and the formation. The measurement electrode is connected to the guard electrode by means of a current transformer, providing a negligible impedance between guard and current electrode, thus allowing for a current to flow through the current electrode into the formation. This current is then measured with the current transformer and associated electronics.
FIG. 4 (prior art) is a schematic circuit diagram of prior art resistivity measurement devices. Zg represents the impedance from the guard electrode to the return electrode, Zc the impedance from measurement electrode to the return electrode, and Zm the mud channel impedance between measurement electrode and the guard electrode. The measurement transformer impedance between measurement electrode and the guard electrode is assumed to be negligible compared to Zm, so that there is virtually no current flow through the mud between the measurement electrode and the guard electrode.
With the measured center current Ic and the (also measured) voltage source output voltage V0, the measurement electrode impedance Zc can be calculated:
                              Z          c                =                                            V              0                                      I              c                                .                                    (        1        )            All the parameters in Eqn. (1) may be complex numbers.
There are two problems with the prior art configuration that have not been addressed in the past. Firstly, the signal-to-noise ratio of the galvanic resistivity imaging tool is largely dependent on the current amplifier equivalent input voltage noise, which limits the precision of the measurement system for a given electrode voltage. Conversely, the required signal-to-noise ratio determines the electrode voltage.
Secondly, since currents in the guard electrode and the measurement electrode are substantially independent, there can be several orders of magnitude difference between these currents. The magnetic field generated by the current in the guard electrode couples into the current measurement transformer TX1 and leads to severe crosstalk. This can result in large measurement errors depending upon the ratio of currents in the guard electrode and the measurement electrode. This results in the dynamic range of the measurement system becoming a function of this current ratio. The present invention addresses these two problems to give an improved method and apparatus for determination of resistivity images of earth formations.